A magnetic compass which is installed in a vehicle must be calibrated in the vehicle to account for the disturbing effect of the vehicle on the earth's magnetic field. It is well known that automotive land vehicles, as well as boats and aircraft, produce a magnetic field due to the presence of ferromagnetic materials, electrical current carrying wires and the like and this magnetic field interferes with the earth's magnetic field at locations within and adjacent the body of the vehicle. The magnetic field sensor of a magnetic compass responds to the ambient or local magnetic field within which it is immersed for the purpose of direction finding with reference to the earth's magnetic field. The magnetic field vector produced by the vehicle, herein referred to as the deviating magnetic field vector, combines with the earth's magnetic field vector to produce a resultant magnetic field vector which, without calibration or compensation, is unsuitable for reliable and accurate direction finding. The resultant magnetic field vector must be compensated for deviation in order to obtain accurate direction finding by a magnetic compass.
It is known in the prior art to provide deviation compensation in a magnetic compass by a pair of compensation coils which are energized with current to generate a magnetic field which is equal and opposite to the deviating magnetic field. Deviation compensation of this type for an automotive vehicle compass is disclosed in the Dinsmore U.S. Pat. No. 4,402,142 granted Sep. 6, 1983. This type of deviation compensation is also disclosed in the Schierbeek et al U.S. Pat. No. 4,862,594 granted Sep. 5, 1989. In this method of deviation compensation, the vehicle must be oriented with a heading aligned with one of the four cardinal directions relative to magnetic north with coil current adjustment being made to obtain a compass indication corresponding to the cardinal point and then the vehicle must be reoriented to a heading corresponding to another cardinal point at right angles to the first and the current in the corresponding compensation coil is adjusted to obtain compass indication of the second orientation. This procedure for deviation compensation is usually carried out by the vehicle driver and even though it may be automated in a computer controlled compass, it results in inaccuracy unless the vehicle heading is accurately aligned relative to magnetic north.
Another method of deviation compensation for magnetic compasses in vehicles is also known and is referred herein as the one hundred eighty degree compensation method. In this method, the magnetic field which is the resultant of the earth's field and the deviating field of the vehicle is measured with the vehicle in a selected orientation relative to magnetic north and then the resultant field is measured with the vehicle in an orientation one hundred eighty degrees from the first. Using the measured values of the magnitude and directions of the resultant fields, the deviating field is calculated in both magnitude and direction. The calculated value of the deviating field is stored and subtracted from the magnetic field measurements subsequently taken by the compass in use for direction finding to thereby compensate it for deviation. This method of deviation compensation for a flux gate compass is disclosed in the Bauer et al U.S. Pat. 4,733,179 granted Mar. 22, 1988 and in the Hormel U.S. Pat. No. 4,720,992 granted Jan. 26, 1988. The one hundred eighty degree method for deviation compensation has been known for a long time for use with flux gate compasses for aircraft as shown by the Baker et al U.S. Pat. No. 3,683,668 granted Aug. 15, 1972.
Heretofore, magnetic rotor compasses, i.e. those having a magnetic rotor have required orientation at a known angle relative to magnetic north to obtain deviation compensation. It is desirable in vehicle compasses to provide a magnetic rotor compass with the advantages of the one hundred eighty degree method of compensation.
A general object of this invention is to provide an improved method of and apparatus for compensating a magnetic rotor compass for deviation and to overcome certain disadvantages in the prior art.